Research in the properties of carbon molecules during the mid 20th century and during more recent years has led to a wealth of discovery related to allotropes of carbon and the novel material properties that are manifested when carbon atoms are arranged as sp2 hybridized carbon molecules such as C60 and C70. The carbon molecules of particular interest are those arranged in regular ordered hexagonal, pentagonal, or similar arrays as a result of high temperature processing of vaporized carbon. Such regular structures can also be naturally occurring and are produced, although in an uncontrolled and therefore irregular manner, in such environments as the soot from ordinary flames or from burning organic fuel products such as ethylene, methane, benzene or the like.
The resulting ordered carbon structures known as “Buckminsterfullerenes” or simply “fullerenes” or “buckyballs,” were named based on the resemblance of the spheroidal carbon structures to the geodesic structures attributed to the mid-century architect and futurist Richard Buckminster Fuller. Fuller's novel structural designs provided a maximum of strength using a minimum of material and an arrangement that allowed stresses to be easily distributed in an optimized mechanical matrix. Carbon fullerene structures mirror Fuller's designs and thus naturally posses many of the strength maximizing characteristics of Fuller's geodesic structures. While fullerenes are spherical in shape, a multitude of other ordered structural forms exist one of which is the carbon nanotube structure.
Carbon nanotubes belong to the family of buckyball or fullerene structures and are often referred to as “fullerene tubes” or “buckytubes.” Carbon nanotubes can be single walled or multiple walled and posses extraordinary mechanical strength and are extremely efficient in conducting heat. Single walled carbon nanotubes are one of the strongest known materials, with a tensile strength that can be around two orders of magnitude greater than that of, for example, high carbon steel. Further, carbon nanotubes have an extremely high modulus of elasticity. Given the low density of the carbon nanotube structure, the specific strength of the carbon nanotube is the highest of known materials. Further, depending on the exact molecular configuration, including the number of defects in the lattice structure, carbon nanotubes can act as an excellent conductor, a semiconductor, or a semimetal. For single walled carbon nanotubes, a typical diameter is 1 nanometer, but can range from 0.3 nanometers to several nanometers, with a length on the order of centimeters. Thus, the carbon nanotube has a size factor and electrical characteristics that far surpass present limits for micro electromechanical form factors used in circuit fabrication.
Beyond the attractive electromechanical properties however, carbon nanotubes possess attractive quantum properties, the most significant of which for electronic design is the extraordinarily high electron mobility. Carbon nanotubes demonstrate the highest electron mobility at room temperature of any material, being in the vicinity of 100,000 cm2/v-second. The high degree of electron mobility or high electron speed, gives rise to the ability to achieve extremely fast signal rates needed for high frequency applications. Further, high electron mobility allows the high signal performance factors to be achieved at low power levels with a low noise factor.
Accordingly, the desirability of single walled carbon nanotubes or “nanowires” for use in electronic circuits has been widely recognized. Such recognition goes beyond the use of carbon nanotubes simply as conducting structures however. Since nanotubes can be engineered with intramolecular properties, nanotubes of differing properties can be combined to form devices. It will be appreciated that throughout the remainder of the present disclosure the term nanowire will be used. However, the terms nanotube and nanowire will be considered interchangeable. Further, when referring to a nanowire, reference is being made for illustrative purposes to a single walled carbon nanotube.
Difficulties arise in producing carbon nanowires having regular and controllable mechanical and crystalline characteristics required of electronic circuits in a cost effective manner. Present methods of producing carbon nanowires are expensive and do not result in nanowires with the desired characteristics for electrical circuits. One problem facing circuit designers is controlling the placement of the carbon nanowires on a substrate. Since nanowires cannot be produced as films as in traditional integrated circuit fabrication processes, they must be manually placed once grown. Manual placement is painstaking and time consuming. Another problem facing circuit designers is the ability to control the electrical properties of the carbon nanowire. Since the electrical characteristics of the carbon nanotube or nanowire depend on the chirality, or orientation of the molecular lattice with respect to a normalized cylindrical orientation of the tube itself, control of the lattice orientation is crucial to controlling whether the carbon nanowire acts as a strong conductor, a semiconductor, or the like. The most desirable orientation for high electrical conductivity is the so called “arm-chair” orientation, where the parallel sides of the hexagonal structures are aligned so as to be perpendicular to an axis of the normalized cylinder.
Producing carbon nanowires can be accomplished though such processes as arc discharge, laser ablation and chemical vapor deposition (CVD). In arc discharge production, carbon and a small concentration of a Group VIIIb transition metal are simultaneously evaporated. The yield of the arc discharge method is low and the degree of size and structural variation is high between individual nanowires. In laser ablation, a graphite substrate doped with transition metal atoms is vaporized with a laser to produce clusters of nanowire structures. The yield associated with laser ablation methods is better than with arc discharge methods, however the population of nanowire structures grown with laser ablation still possesses a high degree of variation and the clusters tend to be tangled. Further, laser ablation methods tends to require high amounts of energy and are thus undesirable for use in connection with mass production.
CVD can be used to produce carbon nanowires by catalytic decomposition. In CVD based catalytic decomposition, metal particles on a substrate are used to nucleate the growth or precipitation of carbon atoms from a portion of the particle, however, challenges remain for producing carbon nanowires having a degree of uniformity and alignment as CVD catalytic decomposition methods tend to produce imperfectly formed nanowires. Some improvements in alignment can be achieved with CVD processes by the use of plasma generation. In a plasma reactor, the strong electric fields used to generate the plasma can be used to an extent to align the resultant nanowires, however the alignment produced by the application of the plasma generating field is vertical and thus, while interesting for the study of electronic emissions from the tips of such nanowires, vertical alignment is of little use for circuit formation.
Problems with horizontal growth of nanowires arise from the tendency of the carbon nanowires to nucleate from a catalyst particle on a surface of, for example, a substrate, and grow vertically away from the substrate into the carbon vapor rich gaseous environment of the reaction chamber. In an attempt to control the horizontal growth of nanotubes in a circuit, U.S. Pat. No. 7,115,306 11, issued to Jeong et al. on Oct. 3, 2006, the contents of which are incorporated herein by reference, describes a method of producing holes in a sidewall of an aluminum layer and providing a metal catalyst layer at the bottom of the holes. The resulting carbon nanotubes that are grown through CVD or plasma enhanced CVD (PECVD) have a diameter corresponding to the hole diameter and grow in a horizontal direction beyond the hole. Some problems arise with the above described method in that, for example, the diameter is limited by the diameter of the holes, which is described to be from several to several tens of nanometers. However, the production of small nanotubes may be limited by the ability of the metal catalyst layer to be deposited in the bottom of the hole. In other words, small holes my prevent the deposition of the metal catalyst in the hole bottom.
Additional methods exist for controlling nucleation of nanowires for example as described in U.S. Pat. No. 7,052,668 B2 issued to Smalley et al., the contents of which are incorporated herein by reference. In Smalley, the size of catalyst clusters or catalyst precursors in a reaction zone can be controlled by controlling the chemical composition, temperature, and the like of a supercritical fluid used to introduce the catalyst or catalyst precursors, thus controlling the diameter and homogeneity of the resulting nanotubes. Typical problems arise in connection with the nanotubes produced by the method of Smalley in that, while nanotubes of controlled size and diameter can be produced with a high yield, the nanotubes are not grown in situ and must still be collected, separated and placed in their eventual position of use, leading to tedious manual or mechanical handling.
It would therefore be desirable for a method of producing nanowires that could provide horizontal in situ growth and control over parameters of the carbon nanowires such as the diameter, the alignment direction, the crystallinity and the like. The exemplary method would also preferably be a high yield process capable of being performed using, to the greatest extent possible, existing process equipment such as CVD reactors and the like.